ploop - container in a file

When dealing with container filesystems, some problems arise: How to limit the amount of data a container uses, since quota solutions that are container aware are not there yet. Also, when migrating the container to another destination, one usually remakes the filesystem by copying over the files (unless shared storage is assumed). This means that inode numbers are not preserved and also creates a bad I/O pattern of randomly copying over a zillion of tiny files, much slower than copying image of block-device (seq I/O).

Another problem with all CTs sharing host fs is non-scalable journal on host fs: as soon as one CT filled the journal, other CTs should wait for its freeing.